This invention relates to a method and a timing device, wherein the time interval to be measured is determined by counting a preselected number of periods of a time basis, for instance of a RC-oscillator or voltage-controlled oscillator. All these prior timing devices have limited accuracy and stability. Particularly the time measurement depends on the operating voltage of the time basis. Devices of this kind need a relatively high number of separate elements such as condensers and resistors so that they cannot be integrated to a substantial degree.
In order to overcome said difficulties resulting from instabilities of the operating voltage, it has been suggested to provide a time basis comprising a potentiometer connected to a reference voltage source, the reference voltage and the voltage at the cursor of the potentiometer being fed to input terminals of an A/D converter and this converter delivering a digital output value corresponding to the ratio between said reference voltage and the voltage at the cursor of the potentiometer (GB-A-2 159 646). This ratio and digital value respectively is used as a time basis. It is highly independent from the operating voltage because the dividing ratio of the potentiometer does not depend on the operating voltage. However, the prior device disclosed by GB-A-2 159 646 has a microcomputer controlling the timing circuits, this rendering difficult an easy integration of the device. Further, the voltage dividing ratio and corresponding digital value respectively are detected once and memorized for further time counting. However, this results in low stability and accuracy of the time measurement. Experience shows that low cost microprocessors in an industrial environment have to be protected against electromagnetic distubances at all inputs by additional components, i.e. resistors and/or capacities, which will increase the size and the costs of a timing device considerably.